Fluorescent lamps are used as light sources in a wide variety of applications. These applications include consumer and industrial applications, such as home and office lighting. Fluorescent lamps are also used in a number of more demanding applications, for example, for backlighting displays, such as liquid crystal displays (LCDs) and active matrix liquid crystal displays (AMLCDs). LCDs and AMLCDs are used in a variety of products including aircraft flight instruments and portable computers.
A fluorescent lamp, especially when used for backlighting an LCD and an AMLCD in an aircraft application, particularly a military aircraft application, should have a wide luminance range. In addition to having a wide luminance range, the fluorescent lamp should be dimmable to a low luminance level so that a pilot or other user can view the display screen easily under both bright and dark conditions, including night vision goggle (NVG) conditions. Further, the light output of a fluorescent lamp, especially when used for backlighting an LCD or AMLCD in a military aircraft application, should reach its optimum operating level shortly after the lamp is turned on in cold climates. Achieving these two goals has been difficult, as will become apparent from the following discussion.
A typical fluorescent lamp includes a glass tube that contains a gas mixture of mercury and one or more rare gasses, such as argon and neon. A pair of internal electrodes are located inside the glass tube, spaced apart from each other along the length of the tube. The interior wall of the glass tube is coated with a phosphor material. Various ways of causing the internal electrodes to emit electrons in the glass tube are available. For example, a high AC voltage may be applied across the internal electrodes to cause an arc discharge that results in the release of electrons (cold cathode tube). Alternatively, or in addition, if the internal electrodes are in the form of filaments, a filament current may be applied to both internal electrodes to thermionically excite the electrodes to emit electrons (hot cathode tube). The released electrons driven by the applied high AC voltage excite the gas mixture, ionize some gas molecules, and trigger an arc discharge across the internal electrodes, i.e., electric conduction occurs between the internal electrodes. The mercury atoms in the gas mixture are excited to upper energy levels, and some of them emit ultraviolet (UV) radiation when returning to their ground state. When the UV radiation strikes a phosphor coating deposited on the interior wall of the glass tube, the phosphor produces visible light.
Lumination is controlled by controlling the output of the power supply that causes the arc discharge current. Amplitude or pulse width control can be used. Pulse-width modulation (PWM) controls how often the arc discharge current flows, whereas amplitude control controls the magnitude of the arc discharge current. FIG. 1 illustrates the waveform of three arc discharge currents and, hence, the light output. The first and second arc discharge currents 11 and 12 have high and low amplitudes, respectively. Both are continuous AC sinusoids. The third arc discharge current 13 is a pulse-width modulated version of the first arc discharge current. The first arc discharge current 11 produces a bright output. The second arc discharge current 12 produces a dim output. The third arc discharge current 13 also produces a dim output.
At any given frequency, the range of amplitude control is limited at the low end by the minimum level of voltage required to sustain an arc discharge. Operation below this level requires the use of a reignition pulse to provide a minimum level of ionization. For example, in a pulse-width modulated (PWM) dim mode of operation, the ionized species in the gas mixture, such as Hg.sup.+, Ar.sup.+, and e.sup.- that are necessary for stable discharge operation, decay rapidly during the inactive periods between pulse cycles. The ionization decay time is approximately 100 microseconds, as compared to a typical pulse period of 8 milliseconds. Therefore, a reignition pulse is needed to provide a minimum level of ionization in the gas mixture prior to arrival of the next group of excitation pulses. However, the reignition pulse and the resulting ionization create light. Even the smallest reignition pulse, reduced to the minimum pulse width necessary to ionize the gas mixture, creates light that is brighter than the minimum luminance level typically required for dim operation. As a result, it has been difficult to extend the lower limit of the dimming range of a fluorescent lamp.
One approach to lowering the dimming rage of a fluorescent lamp is found in U.S. Pat. No. 5,420,481 to McCanney. As illustrated in FIG. 2A, McCanney proposed the use of a pair of external conducting plates 4, located on opposite sides of a fluorescent lamp tube 5. The pair of external conducting plates 4 produce a transverse electric field through the tube 5. The transverse electric field produces a low-intensity transverse discharge across the plates 4, and maintains a minimum level of ionization in the gas mixture. This eliminates the need for the use of reignition pulses and, thus, extends the lower limit of the lamp's dimming range. Alternatively, as illustrated in FIG. 2B, a pair of external wires 6, attached to opposite sides of the glass tube 5, can be used to create a transverse electric field. Further alternatively, as illustrated in FIG. 2C, a printed wiring board (PWB) 7 including a pair of conductive traces 8 along the tube 5 can be used to create a transverse electric field. The McCanney devices, however, suffer some limitations. It is difficult to secure plates, wires, or a PWB to a lamp having a curved or serpentine shape. (Serpentine-shaped lamps are ideally suited for use in AMLCD and LCD backlights). It is particularly difficult to arrange the wires or the conductive traces on a PWB to precisely follow a complex lamp tube geometry. For example, in the case of PWB electrodes, the efficiency of the electric field ionization is dependent on the proximity of the conductive traces to the glass tube. Since a glass tube is typically bent into various forms by hand, it is extremely difficult to exactly align the glass tube with the printed traces on a wiring board. When close and consistent alignment is not achieved, higher voltages are required to produce a transverse electric field adequate to produce a transverse discharge. Further, the intensity of the discharge will vary along the length of the discharge. Furthermore, it is difficult to handle lamps having plates, wires or a PWB with conductive traces during manufacturing. Thus, a need exists for a fluorescent lamp with an extended lower limit dimming range that is easy to handle during the manufacture of products incorporating the fluorescent lamp, and that provides a uniform low intensity luminance level throughout the length of the lamp.
Another challenge associated with fluorescent lamps is the need to warm up the tube of the lamp in order to reach the lamp's optimal light output level. This challenge is particularly difficult to meet in fluorescent lamps intended for use in products designed for operation in cold climates, such as LCD and AMLCD instruments designed for use in military aircraft intended for possible use in arctic regions. More specifically, the light output of a fluorescent lamp depends on the mercury vapor pressure within the lamp's glass tube, and the mercury vapor pressure varies depending upon the temperature of the glass tube. FIG. 3 shows that, for a fluorescent lamp having a small diameter glass tube, such as 15 mm, the optimum temperature for maximum light output is about 50.degree. C. When the temperature is below the optimum temperature, mercury atoms are condensed on the wall of the glass tube and/or other cold internal surfaces of the lamp, such as the electrode leads. As a result, the mercury vapor density within the glass tube decreases. As the mercury vapor density decreases, the UV radiation production rate decreases. Hence, the visible light output from the lamp decreases.
One method of increasing mercury vapor pressure is to increase the wall temperature of a fluorescent lamp's glass tube. In the past, this has been accomplished by passing an electrical current through a resistive, small-diameter heater wire wrapped around the exterior of the glass tube. The application of the resistive wire is typically accomplished by winding the wire in a spiral fashion along the length of the glass tube. Such winding becomes complicated when the glass tube has a nonlinear configuration, such as a serpentine configuration, particularly where the glass tube bends. Further, the point contacts that occur between a resistive wire wrapped around a glass tube and the glass tube result in poor heat transfer between the wire and the glass tube. In addition, in order to prevent the wire from unraveling from the glass tube, an adhesive is typically applied over the wire at periodic intervals along the glass tube. The adhesive further diminishes the rate of heat transfer between the wire and the glass tube. As a result, more power than desired must be applied to the wire to raise the temperature of the glass tube to the desired level. Furthermore, from a manufacturing viewpoint, it is difficult to bond a resistive wire to a glass tube such that the wire is in intimate contact with the tube. Thus, a need exists for a fluorescent lamp design having a heater that has a high heat transfer rate and is easy to manufacture.
The present invention is directed to providing a fluorescent lamp with an extended low end dimming range and rapid warmup capability that is easy to handle during the manufacture of products incorporating the fluorescent lamp. While primarily designed for use in the backlights of LCD and AMLCD displays designed for use in low-temperature environments, such as AMLCD and LCD flight instrument displays designed for use in military aircraft, fluorescent lamps formed in accordance with the present invention may also find use in other environments.